The present invention relates generally to the casting of molten metals and, more particularly, to the refractory elements used to control the flow of molten steel during continuous casting operations, for example.
One important object of the present invention is to provide a new material for the molten metal contacting surfaces of casting elements. More particularly, it concerns casting elements such as slide gate valves and nozzles for casting aluminum killed steels or ferro-alloys containing aluminum.
Following the ore smelting operations and iron-into-steel conversion, the metal contains non-negligible amounts of dissolved oxygen. This oxygen is quite pernicious, notably during casting operations and in general working of the metal. In order to suppress this dissolved oxygen, calming agents are employed. Among the latter, silicon, carbon, magnesium and aluminum are commonly used agents. It is primarily this latter metal, aluminum, alloyed or not, that is used to "kill" or calm steels, since it provides the most effective results.
Nevertheless, during casting operations, for reasons and under conditions that are still poorly known, fine particles of alumina accumulate in the casting shrouds, e.g., such as the nozzles, capable of causing an almost total blockage and a definite problem in the normal development of the casting systems. It is further noted that alumina may also deposit on the orifices of the slide gate valve through which molten metal is teemed from the tundish, for example. Such alumina deposition causes unwanted variations in the flow rate of the molten steel and may also result in eventual total blockage of the valve.
In order to alleviate these difficulties, an initial solution consists in frequently changing the casting elements where alumina accumulates. This solution is expensive and considerably shortens the service life of the casting elements, notably the nozzles which convey the metal emerging from the distributor in the case of continuous casting.
In order to increase the service life of the casting elements, another solution consists in reducing the amount of aluminum to be added as much as possible. This palliative method is however not satisfactory.
At the beginning of the 1980's, studies conducted by Emilio Marino and Aldo Ramacciotti led to the French patent published under U.S. Pat. No. 2,529,540. It is indicated in this document that it is possible to avoid any accumulation of alumina in the nozzles, since these latter are produced from lime, if necessary, with some additive elements. These elements, the sum of which is between 3% and 25% by wt., are designed to promote the growth of the grains.
This technique, which definitely gives good results with regard to the accumulation of alumina grains, has a major shortcoming, that of the necessary precautions in the storage and transport of these nozzles. In effect, such a refractory material, comprised for the most part, even up to 100%, of unhydrated lime, is extremely reactive with respect to humidity, which results in a loss of mechanical strength when such a casting element has been subjected to moisture.
A further prior approach to solve this problem has been taken in which the parts of the casting elements in contact with the molten metal are covered with a mineral layer that forms eutectics having a low melting point with alumina. These eutectics are washed and entrained with the alumina by the flowing of the molten metal. While this technique provides a substantial improvement as compared with the existing techniques, it replaces the accumulation of alumina with an erosion of the alumina anti-accumulation layer, which also reduces the service life of the casting elements.
Use of composite material of the SiAlON type offers a significant advance (cf. D. B. Hoggard, G. I. Rancoule, L. C. Myers, H. K. Park, and M. K. Fishler, "Development of a Liner to Reduce Alumina Buildup in Graphitized Alumina Submerged Pouring Nozzles Used in The Continuous Casting of Steel", published at the Second International Conference on Refractory Materials, "Refractories '87, TOKYO, Japan, Nov. 10-13, 1987"), but not a decisive one. U.S. Pat. No. 4,870,037 of D. B. Hoggard and H. K. Park also discloses a continuous casting nozzle having a liner insert of a SiALON-graphite refractory material for prevention of alumina build-up.
One of the purposes of the present invention is to provide a material for at least the metal contacting surfaces of the casting elements to avoid erosion and alumina accumulation. The present invention thus provides casting elements such as slide gate valves in which erosion due to molten metal is minimized and the accumulation of alumina is difficult, if not impossible.
In addition, the present invention provides a casting process for ferro-alloys killed with aluminum or with one of its alloys, such as the aluminum/magnesium alloys which utilizes the said casting elements.
These, as well as other advantages, are achieved by means of a material for covering the casting elements, characterized by the fact that it has a calcareous composition capable of liberating calcium oxide, under the conditions of use, at a temperature at least equal to 1400.degree. C. due to the fact that its calcium content, expressed in weight percent of calcium oxide, is between about 10% and about 60% and preferably between about 20% and about 50% and due to the fact that the said calcareous composition is bound by means of a binder chosen among the group comprised of those utilizing bonds of the carbon/carbon type by sintering. The material may also have a ceramic bond or may be applied as a fused coating by plasma spraying to the metal contacting surfaces of the casting elements.
During the study that was conducted on the present invention, it was in effect demonstrated that with such a composition, during the casting of a ferro-alloy killed with aluminum or with one of its alloys, such as the aluminum/magnesium alloys, a layer of an eutectic or an eutectoid mixture, the alumina/calcium oxide ratio of which is in the vicinity of 6:1, developed in contact with the metal rich in aluminum. This layer has the surprising property of playing an anti-adhesive role with respect to alumina and an adhesive role with respect to the basic material, perhaps due to the affinity of the latter for calcium. Thus, this layer prevents any accumulation of alumina and by remaining fixed on the casting element itself protects the latter from wear by entrainment with the casting of steel or ferro-alloy.
Thus, after the first casting, provided the ferro-alloy was sufficiently rich in aluminum, the faces in contact with the ferro-alloy that are covered with the said material become covered at least partially with a composition that can be qualified as an eutectoid and whose alumina/unhydrated lime molar ratio is in the vicinity of 6:1.
The host material of the calcium, generally in the form of oxide, should preferably have a sufficient affinity for the lime to assure its stability, even in a relatively humid atmosphere, i.e., that it should avoid a phenomenon abundantly described in the literature of thermodynamic reactions between oxidized products and water, according to which the reaction of formation of slaked lime is a very strongly exothermic reaction of hydration of the unhydrated lime, this reaction being followed by a considerable volumetric swelling (capable of breaking physical bonds by expansion).
The host material should also have the property of being sufficiently little bound to the unhydrated lime and of liberating the latter sufficiently during casting, so that it can combine with alumina and form, possibly with various other impurities, an interfacial phase that can be qualified as an eutectoid.
The host material and the calcareous composition should be refractory at the casting temperature. The calciferous composition should be as homogeneous as possible in the particular range and is preferably realized by melting or by reaction in the solid state of the host material with a calcium compound such as quick lime.
The host material can be used either pure or in the presence of controlled amounts of additive elements.